It is generally known that data communication networks known as optical transport networks or OTNs are mostly based on a ring-based architecture. Rings are typically easy to manage and offer a fast way for protection switching. Unfortunately, they tend to be capacity-inefficient and can require more than twice the working capacity for protection. Mesh-based networks, on the other hand, require much less spare capacity, but have the drawback of complicated protection mechanisms. Therefore, several protection techniques for the transition of ring networks towards mesh-based networks have been considered.
One such protection technique is known as pre-configured protection cycles or p-cycle protection. In p-cycle protection, the spare capacity for span protection is organized in cycles and shared among on-cycle links and straddling links of the cycle. In this way, a redundancy lower than 50% for protection is achievable, while still retaining the speed associated with the use of rings.
Thus, p-cycle protection or restoration has been proposed in mesh networks for achieving ring-like restoration speed with mesh-like capacity efficiency. Ring-speed arises because only two nodes perform any real-time actions for each restored signal unit, and such actions are fully predetermined before failure and are triggered by each node detecting the failure autonomously. The surprising capacity efficiency is the less obvious property but it is ultimately attributable to the aspect of protecting straddling link failures as well as on-cycle failures. This seemingly small difference between a ring and a p-cycle actually leads to major differences in protection capacity requirements.
While the p-cycle scheme promises ring-like restoration speed at mesh-like capacity efficiency, there are many aspects of the p-cycle scheme from a carrier class deployment perspective that have not received much attention so far. These, if not properly addressed, could become a stumbling block for any future deployment of p-cycle restoration in carrier networks.
First, p-cycles assume single link failure. Node failures and multiple link failures can occur in a real network. In the presence of these failures, p-cycle restoration suffers from the same misconnection problem that is solved in a Multiplex Section Protection Ring/Bi-directional Line Switched Ring (or MS-SPRING/BLSR) architecture through squelching. This requires a signaling protocol after failure, which may impact the restoration latency.
Second, p-cycles do not protect against node failures. Node-encircling p-cycles have been proposed to handle this problem. Node-encircling p-cycles may require a higher degree of meshing and additional backup capacity and therefore also reduce capacity savings.
Third, implementing p-cycles in a dynamic setting, where p-cycles are setup as the demand arrives, has to guarantee that contiguous concatenation requirements associated with a Synchronous Optical Network (SONET) architecture or a Synchronous Digital Hierarchy (SDH) architecture are satisfied. This leads to fragmentation and bandwidth constraints around the cycle and again may limit the capacity savings.